Dissatisfied with congress? Scientists try to help.

You want to love a book called "Bonk: The Curious Coupling of Science and Sex," especially because its author is Mary Roach, who has already proven herself on the topic of death in bestsellers "Stiff: The Curious Lives of Human Cadavers" and "Spook: Science Tackles the Afterlife." When you don't love "Bonk," you worry that it's you: You weren't in the mood; you were unresponsive; you couldn't stay in the moment. But maybe not.

"Bonk" is about sex research. What does that mean? It means Alfred Kinsey watching people have sex in his attic; Masters and Johnson developing the penis camera; and friend-of-Freud Marie Bonaparte theorizing, wrongly, that a woman's ability to climax depends on the geometry of genital structures. After rehearsing this history, Roach merrily traverses the globe, visiting Danish pig inseminators and a rogue Egyptian doctor who has published 1,016 papers and conducts his research on prostitutes, not to mention the London lab where she, her husband, and an ultrasound wand engage in coitus.

"Bonk" covers sex machines and vibrators, testicular transplants and penile implants, paraplegic sex and vaginal photoplethysmographs (the "acrylic probe" that researchers use to measure arousal). It also includes a tedious complement of coy asides, almost-dirty jokes, and nudge-nudge-wink-wink illustrations, like the photograph of three hazelnuts that precedes Chapter 7, "The Testicle Pushers: If Two Are Good, Would Three Be Better?"

Roach is good at what she does. She knows how to find information, whether she's burrowing in archives, translating the academese of scientific papers, or interviewing researchers. Her explanations of physical phenomena are clear and informative. I now know that erections depend upon the smooth-muscle tissue in the corpora cavernosa, and that we see only one-10th of the clitoris, somewhat in the manner of an iceberg.

After a while, though, the gleeful anecdotes start to blur, the gadgets start to merge, and you just want to smoke your cigarette and go to sleep (maybe it's impossible to avoid innuendo when writing about sex). To shift metaphorical gears, Bonk has lots of trees but little forest. Structuring the book as a series of reports on individual studies and issues - from the relationship between orgasm and fertility, to the health benefits of masturbation, to the sexual habits of rhesus monkeys - Roach misses the opportunity to ask, and answer, some big questions.

Her introduction signals her approach. After discovering sex research "while procrastinating in a medical school library," Roach immediately started to wonder "about what it must be like, the . . . hassles that the researchers faced - raised eyebrows, suspicious wives, gossiping colleagues." She points out that "even when a researcher carefully explains a sex-related project - its purpose and its value - people may still suspect he or she is a perv." And she notes her own experience with "the cringe factor," as she terms it.

Caught up in the potential for titillation, Roach fails to ask some fundamental questions that could have grounded the book's plethora of narrative and information. What is the rationale for sex research? Could our collective discomfort have as much to do with research as with sex? Can science ever get to the bottom of sex?

The irony is that Roach's subjects clearly understand these issues. They know their cause: improving people's sex lives. One of the original sex researchers, late-19th-century gynecologist Robert Latou Dickinson, "believed that lame sex destroyed more marriages than did anything else, and . . . something ought to be done." Psychologist John B. Watson wrote that sex "is admittedly the most important subject in life. . . . the thing that causes the most shipwrecks in the happiness of men and women." Masters and Johnson moved on from sex research to sex therapy.

But do we really want to admit that sex is that important? In comparison to curing cancer or eliminating world hunger, erectile dysfunction and female sexual arousal disorder score fairly low on the improving-the-world-through-science agenda (if fairly high on the drug-company-profit agenda, another topic Roach never addresses). This, I'd suggest, could be another reason we are so discomfited by the idea of sex research: It uncovers the primacy of our selfish desires, revealing that we are as interested in our orgasms as we are in bettering the human race.

Roach ends "Bonk" by discussing a forgotten Masters and Johnson study revealing that gay men and lesbians in committed relationships have the best sex "because they 'took their time' " (the study has been forgotten not because of its findings, but because it appeared in a book that, despite its findings, advocated converting homosexuals to heterosexuality). The irony here is that taking your time has nothing to do with hormones, pig insemination, or the location of the clitoris. Roach herself points out that "sex is far more than the sum of its moving parts," suggesting the fundamental inadequacy of her topic, but, just as she never follows up on her subjects' big-picture observations, she doesn't follow up on her own.

It's unreasonable to ask that a book be something it's not, but we can still ask what a book is trying to accomplish and whether that is sufficient accomplishment. Roach provides a generally lively, if occasionally smarmy, account of sex research, but she misses an opportunity to help us think about the values we place on research, science, sex, and, especially, good sex.

Rebecca Steinitz is a writer, editor, and consultant who lives in Arlington.